Processes for controlling afterburn in a reheater and for controlling loss of entrained solid particles in combustion product flue gas

Abstract

Processes for controlling afterburn in a reheater and loss of entrained solid particles in reheater flue gas are provided. Carbonaceous biomass feedstock is pyrolyzed using a heat transfer medium forming pyrolysis products and a spent heat transfer medium comprising combustible solid particles. The spent heat transfer medium is introduced into a fluidizing dense bed. The combustible solid particles of the spent heat transfer medium are combusted forming combustion product flue gas in a dilute phase above the fluidizing dense bed. The combustion product flue gas comprises flue gas and solid particles entrained therein. The solid particles are separated from the combustion product flue gas to form separated solid particles. At least a portion of the separated solid particles are returned to the fluidizing dense bed.

Description

FIELD OF THE INVENTION[0001]The present invention generally relates to processes for controlling combustion in a reheater of a pyrolysis system, and more particularly relates to a process for controlling afterburn in the reheater and controlling loss of entrained solid particles in combustion product flue gas during regeneration of a heat transfer medium.DESCRIPTION OF RELATED ART[0002]Pyrolysis is a thermal process during which solid carbonaceous biomass feedstock, i.e., “biomass”, such as wood, agricultural wastes / residues, algae, forestry byproducts, cellulose and lignin, municipal waste, construction / demolition debris, or the like, is rapidly heated to pyrolysis temperatures of about 300° C. to about 900° C. in the absence of air using a pyrolysis reactor. Biomass may be pyrolyzed using various pyrolysis methods, including the Rapid Thermal Process and catalytic pyrolysis. Under these conditions, solid, liquid, and gaseous pyrolysis products are formed. The gaseous pyrolysis pro...

AI Technical Summary

Problems solved by technology

The buildup of ash in the reheater reduces the operating efficiency of the reheater and reduces the volume available to combust “new” ash entering the reheater.

Ash build-up in the reheater is thus undesirable, and therefore its prompt removal from the reheater is desirable.

Unfortunately, if the combustible solid particles are not separated from the combustion product flue gas and returned to the fluidized dense bed of the reheater for combustion thereof, the entrained combustible solid particles may cause “afterburning” of the combustible solid particles in the dilute phase of the reheater or in downstream lines and equipment, rather than in the dense bed.

However, there may be incomplete combustion of the dilute phase flue gas CO to CO2 or incomplete consumption of O2 in the dilute phase.

Either problem also gives rise to afterburning.

Afterburning is exothermic, and either must be quenched by additional injection of the oxygen-containing regeneration gas, or the combustion product flue gas must absorb the heat of combustion, which undesirably decreases the amount of heat transferred to the dense bed.

In addition to the afterburning problem caused by entrainment of the combustible solid particles, a portion of the hot regenerated inert and catalytic solid particles may be lost if not separated from the combustion product flue gas and returned to the dense bed for recirculation as the heat transfer medium or as a catalyst (in the case of the catalytic solids).

However, the capacity of such separators is often exceeded and such outside separators cannot remove ash from the reheater promptly after combusting the carbon in the combustible solid particles and cannot return the solid particles to the dense bed while the solid particles are still in the reheater.

However, these changes have not entirely prevented loss of such solid particles in the combustion product flue gas.

Such loss increases production costs and lowers throughput of regenerated heat transfer medium to the pyrolysis reactor.

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